The above transversality condition \(\epsilon _{a}p^{a}=0\)  then yields
\(\vec{\epsilon }\cdot \vec{p}=0\) , so that we can define two space-like polarization vectors \(\vec{\epsilon }_{i}\)  satisfying
\(\vec{\epsilon }_{i}\cdot \vec{\epsilon }_{j}=\delta _{ij}\) . Note that \(\vec{\epsilon }_{i}\)  \((i=1,2)\)  and \(\vec{p}\)  form a three
dimensional orthogonal basis system as desired [1]}.
